Why I Don’t Sterilize My Seed Starting Mix

When I was new to starting seeds, I ran across advice on the importance of sterilizing both your seed starting mix and the containers into which it went, in order to kill off any pathogens that might be in your mix. This, ostensibly, was to prevent the dreaded damping off. Damping off causes young seedling to rot right at the soil line. The plant simply falls over at the weak point in the stem and dies. Once the plant is infected, there is nothing you can do to save them.

Damping off is caused by a variety of fungus/molds including Pythium spp., Rhizoctonia solani and Fusarium spp.  These fungi are often found in garden soil (I know for a fact that mine contains Fusarium and probably Rhizoctonia). These fungi thrive in cool/wet conditions and can be moved around in soil opn soil-contaminated items (e.g., pots, workbenches, shovels).

So early on, I diligently put my seed starting mix in the oven in a big stainless steel bowl and baked it at low temperatures (google for details) to kill off any pathogens. And I gave my pots a bleach bath for the same reason. (I’ve read that pouring boiling water through your seed starting mix can also be effective for killing off pathogens).

And then, carefully watering my small seedlings with a spray bottle of boiled/cooled water every day, I started to have seedlings die from damping off anyway. Clearly all of my sanitation was ineffective. I then tried things like watering with chamomile tea, which is supposed to be antifungal. It didn’t help.

What I finally realized some time later was that I was WAY OVERWATERING MY SEEDLINGS. Seed starting mixes often contain peat moss (naturally anti-fungal due to low pH by the way). Peat moss really hangs onto moisture. Various molds and fungus spores are in the air. As soon as your soil leaves your oven its exposed to new contaminants. It’s impossible to maintain a sterile medium in a typical household or greenhouse setup for any length of time. An unopened bag of seed starting mix from a nursery or big box store should not be assumed to be sterile either, given its journey to get to you.

So, I stopped watering so much. And about 99% of my damping off issues went away!

I now plant my seeds, water once, drain well, cover, and maybe don’t water again for a full week. Once plants are up, I remove the cover to reduce humidity, and even then, I don’t always water every day. I don’t water until the surface of the soil is starting to look dry. When the plants are still small, the soil can take quite some time to dry out depending on ambient temperatures.

Over time, I’ve greatly increased the amount of soil and the number of pots I need in the spring. Sanitizing all of this material became a huge burden. And because I had mostly eliminated my damping off issues by not overwatering, I decided to see what would happen if I stopped sterilizing everything. I don’t even normally wash out containers from the previous year anymore. I simply knock last years dried dirt and spiders out of them and refill. And you know what? The plants are FINE!

I also don’t freak out if I do see a bit of green growing on the surface of the soil. This is often an algae, and it turns out to be harmless. For whatever reason, some of my plants seem to always have this green film by the time they get planted out into the garden. If the plants are still actively growing and look healthy, I don’t worry about it.

I make my own seed staring/potting mix and my own organic fertilizer. Sometimes (like just yesterday) this mix is from bags of compost and/or peat moss and/or perlite left over from last year that’s been sitting in my greenhouse, opened, for a full year. I don’t do anything to it other than get it damp if needed and shove it through a rough sieve to break it up/aerate it and remove any larger chunks.

And I also always include a bit of my native garden soil in my seed starting mix. Because I figure if that soil is “teaming with microbes” and most of those microbes are good guys keeping the bad guys in check, I’d rather my seedlings be exposed/colonized by these good guys from the very start of their journey. If the focus of our organic gardening pursuits is to build healthy soil with a thriving microbiome, does it make sense to start out our seedlings in a sterile medium? I personally don’t think it does. (I originally got this idea from Eliot Coleman – who includes his own compost in his seed starting mix for the same reasons.)

Side note: This is also my logic for not using antibacterial soaps in my kitchen or on my body. I’d rather encourage a healthy microbiome on my skin and in my house rather than trying to nuke everything in a futile attempt at sterility. (Don’t get me wrong. Sanitizing surfaces that come into contact with food is just good hygiene – I’m not suggesting anarchy here.)

And in the 15+ years I’ve been growing plants from seed, once I realized that overwatering was causing my problems, I’ve almost never lost a plant to damping off or to another soil borne disease while in pots. It just hasn’t been an issue. Your mileage (climate, humidity, disease pressure, soil mix, water source, growing practices etc.) may vary. But if the need to sterilize soil and pots is keeping you from starting seeds yourself rather than buying started plants, I hereby give you permission to NOT sterilize anything. See what happens. Do a side by side test. Just don’t overwater!

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